Composite reflector



Patented June V22, 1937 UNITED STAT-151s COMPOSITE BEFLECTOR Junius D. Edwards, Oaknnont, Pa., assigner to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application October 11,

10 Claims.

'I'his invention relates to improved aluminum reflecting surfaces of uniform high reflectivity and provided with an artificial clear and transparent protective oxide coating.

Various attempts have been made to produce `stable, corrosion-resistant reflecting surfaces on aluminum by providing an aluminum reflecting surface with an oxide coating. However, the many advantages incident to the provision of the reflecting aluminum surface with such oxide coatings are often offset by the fact that the oxide coatings developed on the surface show various markings, streaks, and lines. are in general invisible on the metal surface before the oxide coating treatment but appear during the oxide coating treatment, and they are apparently directly related to the internal structure of the surface of the metal and the working process by which the article has been formed. For example, when a cup-shaped or hemispherical light reflector is formed by the usual drawing operation, the reflecting surface, after oxide coating, may exhibit drawing lines in the form of relatively non-reflective, concentrically arcuate streaks; or ia at aluminum reilecting surface may, upon being oxide-coated, develop definite lines directly traceable to the structure of the metal surface. Furthermore, the oxide coating produced upon the reflecting surface often has a cloudy and translucent appearance and may at times b e colored. These defects materially reduce thevreiectivity of the treated aluminum reflecting surface.

It is an object of the present inventionto prol duce an article having a durable oxide-coated aluminous metal reecting surface which is uni-- 50 sisting of a metallic base portion and an aluminous metal surface portion having a reecting sin'face, when produced by operations involving a reduction in thickness of Athe aluminous metal surface portion of at least 99.5 percent, and 55 treated to form thereon an oxide coating, does These markings o 1933, Serial No. 693,127

not exhibit the objectionable markings heretofore referred to, and upon the further discovcry that if the surface portion consists of high purity aluminum, or an alloy thereof, an oxide coating may be produced which is clear and transparent and which does not` detract materially from the reflectivity ofthe aluminum surface. Thus, a composite metalarticle may be obtained according to this invention having a durable, uniformly reflecting, oxide-coated aluminous metal reflecting surface having a greater and more uniform reflecting power than has heretofore been obtainable in oxide-coated aluminousA aluminous 'metal (aluminum or aluminum base alloy) surface portion. The baseportion may be of any aluminum or aluminum base alloy of the desired physical properties, which may be conveniently united with an aluxdnnous metal sheet by a hot or cold working operation, such as by rolling or drawing. l

In forming the composite metal, any of the known methods for formingsuch materials may be used. i' One method which hasproved par-4 ticularly satisfactory for forming a duplex metal having an aluminous metal base portion anda surface portion of another aluminous metal is to cast the base metal against lthe metal' surface portion in sheetvor other worked form as described in U. S. Patent 1,865,089 to E. H. Dix, issued. June 28, 1932. Another satisfactory vmethod is to bond the aluminous metal surface portion in sheet form with the aluminous metal base portion in sheet or slab form.l The inter mediate composite metal article thus formed must in any case be subsequently worked to pro-V duce a composite metal having an aluminousl metal reflecting surface satisfactory fory the,pur poses of this invention. Tlielworking ofv this intermediate composite metal has the eect of eliminating from the aluminous metal surface portion the internal metallic structure which causes the appearance of lines and streaks whenV the surface portion is subsequently'artificially oxide-coated. 'I'he degree to whichV this bbjectionable metallic structure is removed is dependent upon the total amount of working given Y the aluminousmetal surface portion both b efore and after formation of the composite me- Y tallic article.

It is considered essential that-the composite metal be reduced at least 85 per cent by working the base Ametal and surface metal togetherclas by reducing the composite article in a rolling Regardless of the exact composition of thev aluminous metal surface portion of the composite metal, the markings resulting from the internal structure of the metal which appear upon subsequent oxidation of the surface may'be avoided in the reflector by the use of a material such as y above described. However, if the aluminous metal surface portion be formed of high purity aluminum, that is to say, aluminum of at least 99.7

per cent purity, or certain alloys of such high purity aluminum, the additional advantage is obtained-that vthev oxide coating subsequently formed on the aluminum reflecting surface is clear,A transparent and substantially colgrless, so that an article having a durable reflecting surface of high reflecting power may be produced. In general, the ,presence of a total of 0.1 to 0.3 per cent of most of the usual alloying elements in the high purity aluminum does not materially affect the character of the oxide coating obtained, but amounts greater than this of such elements as silicon, iron and manganese may cause the oxide coating to be cloudy or colored. Certain elements, such as magnesium and zinc, when present in the high purity aluminum alloys in relatively large amounts, do not deleteriously affect the oxide coating formed. In the preferred form of this invention, therefore, the article is formed from a composite aluminous metal comprising a relatively thick raluminous metal base portion having the structural properties desired in theflnished article and a relatively thin worked aluminous metal surface portion consisting of aluminum of 99.7 per cent purity, or a high purity aluminum alloy containing not more than 0.1 to 0.3 per cent-of the usual impurities such as silicon, iron and manganese, and containingnot more than 0.3 per cent of copper in addition thereto. Magnesium and zinc may be present in relatively large amounts as alloying constituents.

The type of reflecting surface desired, whether diffuse or specular, may be produced on the aluminous metal surface portion of this composite metal article by any of the processes known in the art. The production of a diffuse reflecting surface may be accomplished by mechanicalor chemical treatment; care should be taken in this treatment, however, not to remove a 'substantial thickness of the surface coating or expose the base metal. Particularly uniform and bright diffuse reflecting surfaces are obtained by the chemical etching of a composite aluminous metal in which the surface portion is formed of a high purity aluminum alloy containing 0.06 to 0.3 per cent of copper and free from graphitic silicon, in accordance with the invention described in copending U. S. Patent No. 1,999,042, of Junius D. Edwards, CyriiS. Taylor, and Welkerv W.

Wentz, issued April 23, 1935. specular reflect'- ing .surfaces may berproduced by various types of bumng and polishing operations. Y`For example, particularly good specular reflecting surfaces are obtainable by a roll polishing operation Vin which the rolling is carried out with rolls having highly polished surfaces and in which the metal surface is given a. light etching treatment between successive roll passes. Good specular reflecting surfaces are also obtainable by the use of builing operations, but' it is generally necessary or desirable to treat surfaces prepared in this manner in order to brighten them prior to' vbrighteningv may be accomplished by an anodic treatment in a fluoborate electrolyte as described in* the copending U. S. application No. 683,344, of Ralph B. Mason, filed August 2, 1933.

The clear, transparent oxide coating may be produced on the aluminous metal reflecting surface by anodic treatment in various known electrolytes, such as sulfuric acid, or a mixture of sulfuricacid and oxalic acid. For this purpose it is preferable to carry out the anodic oxidation in sulfuric acid,- since coatings of Substantial thickness which are colorless and clear may be obtained, and this result is desirable in order to obtain 4the maximum of protection for the refleeting surface with a minimum of reduction in the reflectivity of the aluminous metal surface. Good protective oxide coatings which are of substantial thickness and which are clear, colorless and transparent may, for example, be obtained ing a 15 per cent solution of sulfuric 'acid as the electrolyte and passing a current having a cur-v rent densityV of 12 amperes per. square inch of anode surface for 10 minutesat about 70 F.

In the accompanying drawing is shown, in Figs. 1, 2, and 3, the improved article of this invention in three successive stages of develop-l ment. Fig. 1 shows a composite metal in sheet form comprising a relatively thick metallic base portion .and a relatively thin aluminous metal reflecting surface portion. In Fig. 2 is shown the same composite metal after a reduction in thickness preparatory to further treatment for the production of an oxide coating onthe aluminous reflecting surface. Fig. 3 shows a completed article manufactured according to this invention, comprising a composite metal having a relatively thick metal base portion, and a relatively thin aluminous metal surface portion, which has been reduced at least 99.5 per cent in thickness in forming the composite article, and which has a bright reflecting surface provided with a clear, transparent oxide coating free from lines or markingsl of any kind.

Having thus idescribed the invention, I claim:

1. A composite metal article having a durable aluminous metal reflecting surface characterized by the absence of structural markings, comprising an aluminous metal base portion and an aluminous metal surface portion provided with a clear, transparent, protective, articial oxide coating, said surface portion having a metallic u structure obtained by working together the base 2. As an article oi' manufacture, a composite metal reflector having a, reecting surface which is characterized by the absence of structural markings. and comprising an aluminous metal base portion and an aluminous metal surface portion provided with a clear, transparent, protective, artificial oxide coating, said composite metal having the structure obtained by reducing its thickness at least 85 per cent by working together the base portion and the surface portion, and the aluminous metal surface portion having a metallic structure obtained by working together the base portion and the surface portion and producing a total reduction in its thickness of at least 99.5 per cent.

3. As an article of manufacture, a composite metal reflector the reflecting surface of which is characterized by the absence of structural markings, comprising an aluminous metal base portion and a high purity aluminum surface portion containing not more than 0.3 per cent of impurities and provided with a clear, transparent, protective, artificial oxide coating, said surface portion having a metallic structure obtained by working together the base portion and the surface portion and producing a total reduction in the thickness of the surface portion of at least 99.5 per cent.

4. As an article of manufacture, a composite metal reector the reflecting surface of which is characterized by the absence of structural markings, comprising an aluminous metal base portion and a high purity aluminum surface portion containing not more than 0.3 per cent of impurities and provided With a clear transparent protective, artificial oxide coating, said composite metal having the structure obtained by reducing its thickness at least 85 per cent by working together the baseportion and the surface portion, and the aluminous metal surface portion having a metallic structure obtained by working together the base portion and the surface portion and producing a total reduction in its thickness of at least 99.5 per cent. p

5. As an article of manufacture, a composite metal reector having a reflecting surface which is characterized by the absence of structural markings and comprising an aluminous metal base portion and an aluminous metal surface portion provided with a clear, transparent, protective, articial oxide coating, said composite metal having the structure obtained by reducing its thickness at least 85 per cent by working together the base portion and the surface portion, and the aluminous metal surface portion having a metallic structure obtained by working together the base portion and the surface portion and producing a, total reduction in its thickness of at least 99.9 per cent.

6. A composite metal reflector having a durable aluminous metal reecting surface characterized by the absence of structural markings, comprising an aluminous metal base portion and a high purity aluminum surface portion containing not more than 0.3 per cent of the elements silicon, iron and manganese in combination, said surface portion having a highly reflecting surface provided with a clear, transparent, protective,

electrolytic oxide coating, said surface portion having the metallic structure obtained by working together the base portion and the surface portion andl producing a total reduction in the thickness of the surface portion of at least 99.5 per cent.

7. The method of preparing a composite metal reector which comprises providing a composite metal ingot having an aluminous metal base portion and an aluminous metal surface portion, working the ingot to produce a total reduction in the thickness of the aluminous metal surface portion of 99.5 per cent, brightening the aluminous metal surface portion of the reduced article,

and applying a clear, protective artificial oxide' coating to the brightened surface.

8. The' method of preparing a composite metal reector which comprises providing an ingot of high purity aluminous metal, reducing the thickness of the ingot to form a slab, casting aluminum alloy base metal against said slab to form a composite metal ingot having a high purity aluminous metal surface portion, working said composite metal ingot to reduce its thickness at least 85 per cent and to bring the total reduction in thickness of the surface portion up to at least 99.5 per cent, treating the surface portion to produce thereon a highly reflecting surface, and electrolytically applying a clear, protective oxide coating to said surface.

9. The method of preparing a composite aluminous metal reiiector having a' durable aluminous metal reflecting surface characterized by the absence of structural markings, which comprises preparing high purity aluminum containing not more than 0.3 per cent of the elements silicon, iron and manganese, and not more than 0.3 per cent of copper, casting an ingot therefrom, reducing the thickness of the ingot to form a high purity aluminum slab, casting an aluminum alloy base portion against said slab to form a composite aluminous metal ingot having a high purity aluminum surface portion, working said composite metal ingot to reduce its thickness at least 85 per cent and to bring the total reduction in thickness of the high purity aluminum surface portion up to at least 99.5 per cent, brightening the surface of the high purity aluminum surface portion, and ,electrolytically applying a clear, protective oxide coating to the brightened surface in an electrolyte containing sulfuric acid. l

10. The method of preparing a composite metal reflector which comprises providing a composite metal ingot having an aluminous metal base portion and an aluminous metal surface portion, working the ingot to effect a total reduction in thickness of the aluminous metal surface portion of at least 99.5 per cent, brightening the aluminous metal surface portion of the reduced article, and applying a clear, protective, articial oxide coating to the brightened surface. y

JUNIUS D. EDWARDS. 

